The research outlined in this application proposes to extend work sponsored by a FIRST Award which has examined the role of ryanodine- sensitive Ca2+ channels (i.e., ryanodine receptors) of sarcoplasmic reticulum in the acute and chronic toxicity of anthraquinones. The long- term goal of the Principal Investigator is to gain a detailed understanding of the mechanisms by which doxorubicin and related quinonoids alter the structure, function, and expression of key triadic proteins involved in excitation-contraction coupling. The cytotoxicity of anthraquinones and naphthoquinones is closely associated with altered cellular Ca2+ homeostasis. What is needed is the identity of the principal macromolecular targets of oxidative damage mediated by quinones and an understanding of the underlying toxicological mechanisms. Excitation-contraction coupling in skeletal and cardiac muscle represents one of the best understood forms of cellular signal transduction and Ca2+ regulation. Specific molecular and functional differences in the way cardiac and skeletal muscle regulate excitation-contraction coupling and intracellular Ca2+ transport have now been defined. Cardiac and skeletal muscle have significantly different susceptibilities to anthraquinone toxicity and provide and excellent model system for understanding the basis of organ selective toxicity involving oxidative mechanisms. The major hypothesis of the proposal address site-selective oxidative mechanisms by which doxorubican and model quinonoids alter Ca2+ regulation across the microsomal membrane of cardiac and skeletal muscle. The comparative nature of the proposal emphasizes how specific molecular and functional differences between excitation-contraction coupling in cardiac and skeletal muscle contribute to the cardioselective nature of anthraquinone toxicity. Doxorubicin-mediated changes in the expression and function of key proteins involved in regulation of microsomal Ca2+ transport will be correlated with the progression of acute and chronic stages of cardiotoxicity in vivo. The major objective's of the proposal are: Utilizing two in vivo rodent models, demonstrate that doxorubicin administered in vivo persistently alters the structure, function and genetic expression of key junctional proteins involved in cardiac, but not skeletal, excitation-contraction coupling and is closely correlated with development and progression of cardiomyopathy. Directly demonstrate major differences in the manner in which the redox state of hyperreactive sulfhydryl moieties on cardiac and skeletal microsomal Ca2+ channel complexes are modulated by allosterically coupled sites for physiological and pharmacological activators and inhibitors. Demonstrate that site-selective oxidation/arylation of cardiac microsomal Ca2+ channel complexes constitute a primary mechanism by which anthraquinones and model quinonoids deregulate microsomal Ca2+ transport and homeostasis in vitro. The comparative nature of the studies will reveal important differences in how cardiac and skeletal muscle utilize redox chemistry to regulate microsomal Ca2+ transport and will establish relevant paradigms for elucidating molecular events linking oxidative insult of microsomal proteins with altered Ca2+ regulation and cytotoxicity.